Use the flow engine for HMergeRemovableSimulatesPhase.

For now, this is a 1:1 reformulation of the block-local analysis, but
this will change.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/294473015

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21452 ce2b1a6d-e550-0410-aec6-3dcde31c8c00